The radionuclide, gallium-67 (Ga67), is of medical importance because it preferentially localizes, by mechanisms poorly understood, in some human soft-tissue tumors. We found by electron microscopic radioautography that Ga67 localizes predominantly in subsellular granules morphologically identified as lysosomes, and notably in macrophages of murine lymphoreticular tissues. Our preliminary experiments in mice indicate that whole-body, acute irradiation results in significant decreases in Ga67 localization in lymphoreticular tissues followed by "recovery" of Ga67 uptake that is dose dependent. We will use light and electron microscopic radioautography combined with appropriate histochemical techniques for lysosomal enzymes. This research will contribute new and important information; it will: (1) demonstrate effects of irradiation and Ga67 distribution and localization in tissues of laboratory animals that will be of immediate practical value to clinical and experimental trials of Ga67 for diagnostic use, (2) test an hypothesis of determining irradiation damage, using Ga67 as a subcellular "marker," that may indicate, on a morphologic level, hitherto unknown biochemical lesions in specific cell types, e.g., macrophages, that result from surprisingly low irradiation exposures, and (3) show ultrastructural changes in cells of lymphoreticular tissues after chronic, low dose rate gamma-irradiation.